This invention relates to electrical circuits and methods used therein for controlling operation of electromechanical relays whose contacts control application of alternating current to an electrical load.
It is known in the art to embody microcomputers in various control systems such as in control systems for controlling heating and cooling apparatus. Typically, the microcomputer in such systems is synchronized with the applied AC (alternating current) power source for determining when, in the applied AC sine wave, the microcomputer will provide a particular output signal. For example, the structure of the microcomputer chip may be such that the microcomputer will provide a particular output signal when the applied AC sine wave is at its zero crossover point and increasing. Regardless of whether the determined time is at zero crossover or at some other specific time in the applied AC sine wave, the microcomputer will provide such signal repeatedly at the same determined time. While this manner of operation may be satisfactory for some functions, it creates a potential problem when used to control operation of electromechanical relays whose contacts control application of the AC source to an electrical load, and more particularly, to an electrical load which is highly inductive.
Specifically, when relay contacts initially close or make a circuit to an electrical load, they generally bounce to some degree. Such bouncing, wherein the contacts rapidly make and break the circuit, causes an electrical arc to be generated between the contacts. The relative strength of the arc is dependent upon the values of the voltage between and current through the contacts at the time the contacts break the circuit. It is noted that when the electrical load is an inductive device such as a motor, the starting current, which appears across the relay contacts when the contacts initially make, can typically be approximately two and one-half times greater than the running current. Thus, when the relay contacts control such a device, the resulting arc due to contact bounce can be appreciable. Furthermore, when relay contacts open or break a circuit to an electrical load to terminate energizing of the load, an electrical arc is also generated. Again, the relative strength of the arc is dependent upon the values of the voltage between and current through the contacts at the time the contacts break the circuit.
If the relay coil is energized in such a manner that the contacts make and break the electrical circuit at the times in the applied AC sine wave when the voltage and current are at their values for maximum power generation, a maximum strength arc is produced. If the relay contacts repeatedly make and break at such times when the arc is at maximum strength, the contacts can eventually weld together. In many relay applications, the relay is but one component of a multi-component control device and cannot be readily replaced. When the relay in such a device fails, the entire multi-component control device must be replaced, thus resulting in considerable cost to the user.
Thus, it is desirable to provide an electrical circuit and/or a method of operation which will minimize the tendency of the relay contacts to weld together. One approach in this regard has been to provide for energizing of the relay coil at a particular time in the AC sine wave which, as empirically determined, will result in the contacts making at the most favorable time in the AC sine wave, namely, when the values of the voltage across and current through the contacts are at their values for minimum power generation. Such an approach allows for the inherent time delay between the time the relay coil is energized and the time the contacts actually make. Such an approach also allows for a change in the time delay. Such time delay can change due to, for example, a change in the mechanical spring forces associated with the relay armature which actuates and/or carries the contacts. While such an approach appears satisfactory, it has a disadvantage of requiring a large number of circuit components to effect this function, thus resulting in a relatively expensive arrangement.
Another approach has been the use of a protective snubber circuit which is connected across the relay contacts. Such a snubber circuit, generally consisting of a resistor and a capacitor, adds considerable cost to the product, especially when there are more than one set of relay contacts to be so protected.
Another approach has been the use of a random cycling routine incorporated in the program of a microcomputer. In such a routine, the time in the applied AC sine wave at which the relay coil is energized and/or de-energized is randomly determined. Due to the inherent nature of random cycling, the effect thereof is unpredictable. Also, a random cycling routine requires a large amount of software code, thus possibly requiring the use of a more expensive microcomputer chip than would otherwise be required.